Regulatory and protective functions have been attributed to murine natural killer (NK) cells in a number of infectious diseases including listeriosis. We have developed an in vitro model to study parameters underlying the activation of naive NK cells using heat-killed Listeria monocytogenes (HKL) as stimulator. Independent from expression of the cell surface marker NK1.1, NK cells lysed YAC-1 cells after in vitro stimulation with HKL or HKL + Interleukin (IL)-2, but not medium or IL-2 alone. In contrast, NK cells from severely immunocompromised SCID or RAG-1-/-mutant mice failed to respond to HKL alone, but required exogenous IL-2. Using single-gene-disruption mutant mice, we show that NK-cell activation can be supported by either T-cell receptor (TCR) alpha beta cells, TCR- gamma delta cells. MHC class I or MHC class II gene products. We conclude from these data that recognition of listerial components alone is insufficient for activation of naive NK cells, and that additional costimulatory signals are necessary. These can be provided by various lymphoid cells and appear to be cytokines.